1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method in which after a latent image on a latent image carrier is developed by liquid developer, an obtained visualized image is electrostatically transferred on a recording body through an intermediate transfer body.
2. Description of the Prior Art
Conventionally, a liquid type image forming apparatus which utilizes liquid developer containing a toner and carrier liquid for a development of a latent image has been well known. This liquid type image forming apparatus can utilize a far smaller diameter of a toner compared to a dry type image forming apparatus which conducts a development by a powered toner, so that the liquid type image forming apparatus can form an image which has an excellent reproducibility of dots and a high resolution.
In recent years, an improved liquid type image forming apparatus has been proposed. In the improved liquid type image forming apparatus, the visualized image which is developed on the latent image carrier is not directly transferred onto the recording body such as a transfer paper or the like. The visualized image is first transferred onto the intermediate transfer body, and then the image is retransferred to the recording body (For instance, Japanese Patent Laid-Open 2001-337572). This liquid type image forming apparatus causes the intermediate transfer body such as an intermediate transfer belt or the like to closely fit with the recording body with deforming the intermediate transfer body flexibly in accordance with a delicate irregularity on the surface of the recording body. Therefore, the visualized image can be transferred onto a recording body which has a rough surface such as a heavy paper or the like. It is also possible to form a full-color image by transferring an overlapped visualized image, which has a different color each other, on the intermediate transfer body.
Moreover, a liquid type image forming apparatus which includes a heating device for heating the intermediate transfer body is described in Japanese Patent Laid-Open Hei11-167295.
However, this heating device heats the intermediate transfer body in order to heat transfer the toner image on the transfer body from the intermediate transfer body. The device is not for heating the intermediate transfer body by a different purpose of transferring with an electrostatic transfer method which does not require the heating of the intermediate transfer body.
Conventionally, an image forming apparatus, which adopts a transfer device for secondly transferring the image on the intermediate transfer body onto a transfer material such as a paper for transferring or the like after the image on the latent image carrier is primarily transferred on the intermediate transfer body by utilizing the intermediate transfer body, has been known. The transfer device, which utilizes this intermediate transfer body, has been widely used for a color image forming apparatus. In the color image forming apparatus, several color images which are formed for each color component on the latent image carrier are transferred sequentially on the intermediate transfer body with overlapping the images, and then the overlapped image is transferred together on the transfer paper so as to form the color image. The transfer device which utilizes the intermediate transfer device has an advantage which can form the color image stably on the varied transfer materials including the heavy paper. Furthermore, if an elastic material is used as the intermediate transfer body, it is advantageous for a transfer material, which has a rough surface, to obtain a fine transfer performance.
As a developing device which forms the image on the latent image carrier, a developing device which utilizes a solid developer and a developing device which utilizes liquid developer have been widely known. In the developing device which utilizes the solid developer, an electrostatic latent image is developed on the latent image carrier by a coloring powder of the developing device so as to form a powdered image on the latent image carrier. However, when this powdered image is transferred by the transfer device having the intermediate transfer body, there is a problem that a filming is generated on the intermediate transfer body by the component of the powdered image. Therefore in Japanese Patent Laid-Open 2000-275988, the present inventor proposes a color image forming apparatus in which a process for primarily transferring the powdered image onto the intermediate transfer body is performed at plural times in accordance with the number of necessary colors, and secondary transfer is performed for making a transfer medium abut by a secondary transfer roller in accordance with a powdered image on the intermediate transfer body after the powder image is formed on the intermediate transfer body and transferring the powder image from the intermediate transfer body to the transfer medium. In this case, it possesses a removing mode to remove a foreign matter stacked to the intermediate transfer body, and a secondary transfer roller and the intermediate transfer body are slid at circumferential speed different from that of the secondary transfer roller when the secondary transfer to transfer the powder image from the intermediate transfer body to the transfer medium is executed at the time of operation in the removing mode.
On the other hand, the developing device which utilizes the liquid developer is advantageous for obtaining a sharp image because the diameter of the toner is smaller compared to the device which utilizes the solid developer. Therefore the use of the liquid developer has been reconsidered in the recent years. The liquid developer is generally a developer in which a toner having a major component of coloring agent made of carbon black, organic pigment, or dye and binding agent made of resin is dispersed into the carrier liquid having a major component of solvent of a high insulation performance and a low dielectric constant.
As mentioned above, the liquid type image forming apparatus which includes the intermediate transfer body, the visualized image can be transferred onto the recording body having the rough surface such as the heavy paper or the like.
However, in this liquid type image forming apparatus, the following problems have been left. In other word, in a developing position where the latent image carrier and the developer carrier of the developing device face each other, a thin layer of the liquid developer carried by the developer carrier develops the latent image in such a manner that the toner in the liquid developer is migrated toward the latent image, and the toner is gathered. The liquid developer which constructs the visualized image after developing includes a large amount of the toner which is gathered as the above mentioned, while severely decreases a ratio of the carrier liquid compared to before developing because a large amount of the carrier liquid is left in the developer carrier of the developing device. In case of a transfer method utilizing the intermediate transfer body, the ratio of the carrier liquid is decreased when the visualized image is transferred from the latent image carrier to the intermediate transfer body. This visualized image decreases the migration performance of the toner because of reasons in which the toner becomes hard to maintain the electric charge. Therefore, the electrostatically transferring from the intermediate transfer body to the recording body becomes difficult, and then a transfer failure is easy to be generated. This transfer failure is further prominently generated by using the liquid developer of a high viscosity and a high concentration (Toner having a concentration of about 5 to 40 [wt %] is contained in the carrier liquid, and the viscosity is about 50 to 5000 [mPa·s] in which the present inventor is developed. Moreover, the transfer failure is prominently generated without adopting a process, which applies pre-wet liquid on the latent image carrier before developing. In order to control the adhesion of the toner onto a non-image portion i.e. a stain of the surface, adopting an arrangement for sweeping the adhered toner contributes to generate the transfer failure prominently because the carrier liquid in the visualized image is absolutely adhered to the electrostatic member. The arrangement for sweeping the adhered toner is to sweep the toner which is adhered onto the non-image portion by contacting the electrostatic member such as a sweep roller or the like to the latent image carrier after developing and before transferring.
In order to prevent the above mentioned transfer failure, it is possible to improve releaseablity between the toner and the intermediate transfer body by providing a surface layer which decreases a friction resistance by containing a fluorine compound.
However, generally the intermediate transfer body is pressed strongly by the latent image carrier to transfer the visualized image, and the intermediate transfer body tends to strongly hold down a cleaning member (for example, a blade) to clean residual developer of transferring. Therefore, even thought the intermediate transfer body includes the surface layer which has excellent releaseability to the toner, the releaseability to the toner is decreased by deteriorating the surface layer gradually. Consequently it has been difficult to prevent the transfer failure for a time.
On the other hand, as mentioned above, in the development device which utilizes the liquid developer, the development is carried out in such a manner that the toner having the electric charge is moved electrostatically in the carrier liquid in the developing portion where the liquid developer carrier carrying the liquid developer and the latent image carrier face each other. At this point, the carrier liquid is adhered to the latent image carrier side from the developer carrier together with the toner. In the primarily transfer portion where the latent image carrier faces the intermediate transfer body, the transfer is conducted in such a manner that the toner having the electric charge is moved electrostatically in the carrier liquid; however, some of the carrier liquid is transferred to the intermediate transfer body from the latent image carrier together with the toner. In the secondary transfer portion, some of the carrier liquid which is transferred at the primary transfer portion is transferred onto the transfer paper from the intermediate transfer body together with the toner. The image is transferred onto the transfer paper by this process. However, the ratio of the carrier liquid, which is transferred by adhering to the transferring toner, is decreased at the time of the secondary transfer compared to the primary transfer. Therefore the solid content ratio of the toner becomes higher. Furthermore, in the secondary transfer, the transfer member often has a nature to absorb the carrier liquid, so that the ratio in the carrier liquid is further decreased, and the solid content ratio of the toner is further increased.
In the developing device utilizing the liquid developer, it is known that the carrier liquid is pre-wet coated onto the latent image carrier before developing. However, if the pre-wet coating is not carried out, the solid content ratio of the toner on the latent image carrier is decreased, and the solid content ratio of the toner on the intermediate transfer body tends to be increased. When a sweep roller or the like is disposed to decrease the ratio of the carrier liquid on the latent image carrier after developing, the solid content ratio of the toner on the intermediate transfer tends to be increased as well.
When the solid content ratio of the toner is increased by decreasing the ratio of the carrier liquid, the toner becomes hard to be migrated. Moreover, in the liquid developer, if the ratio of the carrier liquid is decreased, the toner becomes difficult to maintain the electric potential, so that the toner becomes hard to be migrated electrostatically. Therefore even though the sufficient transfer performance is obtained in the primary transfer, the transfer performance in the secondary transfer may be decreased. Especially, when the liquid developer of the high viscosity and the high concentration in which the toner is dispersed into the high viscosity of the carrier liquid is used, the migration of the toner becomes harder, and the sufficient transfer performance is hard to be obtained in the secondary transfer.
When the secondary transfer is carried out, the transfer performance is improved in such a manner that the image on the intermediate transfer body and the transfer paper are pressed to closely fit by using a member for pressing the transfer member to the intermediate transfer body such as a transfer roller. Especially, if the high pressure is applied for the transfer member which has a rough surface roughness, the toner is transferred onto the concave portion of the transfer member, so that the transfer performance is improved. At this point, high pressure is applied for improving the transfer performance of the secondary transfer.
However, on the intermediate transfer body, as described above, the solid content ratio of the toner is high, so that the viscosity of the developer is also high. Furthermore, some portion of the resin of the solid content in the liquid developer is swelled in the carrier liquid and has an adherence. Therefore, the developer has the high solid content ratio on the intermediate transfer body, so that the resin in the developer is easy to be adhered to the intermediate transfer body. If the resin in the developer is adhered onto the intermediate transfer body, and the body receives the high pressure in the secondary transfer portion, the resin may be firmly fixed for a long time.
After the secondary transfer, the residual toner and some of the carrier liquid which are not transferred onto the transfer body are adhered to the intermediate transfer body. A cleaning blade as a cleaning device for the intermediate transfer body is disposed, and the residual toner and the carrier liquid are eliminated with sliding the intermediate transfer body by the cleaning blade. However, there may be a case that the resin contained in the liquid developer is adhered to the intermediate transfer body and firmly fixed in the cleaning portion. This is because of the resin contained in the liquid developer is softened by the heat and the friction with the cleaning blade.
Recently, in order to improve the dispersibility of the toner into the carrier liquid, in the using environment, developer that the dispersing agent which is made of liquid resin is added in to the liquid developer is used. In the liquid developer which contains the liquid resin, the resin is easy to be adhered to the intermediate transfer body by its viscosity, and the resin is easy to be firmly fixed onto the intermediate transfer body by the friction and the pressure from the secondary transfer portion or the cleaning portion.
If such substance is firmly fixed on the surface of the intermediate transfer body, the function as the intermediate transfer body is lost. Especially, the releaseability from the intermediate transfer body is deteriorated, and the secondary transfer performance is remarkably lowered. Moreover, the image which is developed by the liquid developer has the high solid content ratio of the toner as described above, so that the secondary transfer performance is lowered compared to the primary transfer. Moreover, if the influence by firmly fixing the substance onto the intermediate transfer body is added, the deterioration of the secondary transfer performance is appeared prominently.